In order to automatically extract an arbitrary 3D region from 3D image data (volume data), we have developed a physics-based approach using a 3D deformable grid model, which we called "3D Active Grid". 3D Active Grid is an extension of 3D deformable surface models, such as "balloon", and "3D Active Net". Unlike 3D deformable surface models, the proposed method enables us stable extraction of a 3D region by using both boundary and interior information. Moreover, since the generated geometry data can be input directly into FEM (Finite Element Method), 3D Active Grid has many potential applications including virtual simulation of surgery, shape modeling, computer vision, and so on.